Significance Space flight affects circadian rhythms. It produces a misalignment between the internal body rhythms which may be linked to reduced performance of simple tasks and psychological and sleep disorders. Most people treated for such conditions are female. Objectives This project examines the effects of altered lighting and gravitational environments on the circadian rhythms of body temperature, activity, drinking and various performance parameters (such as hand-eye coordination and short term memory) of male and female rhesus monkeys. Results Our previous studies have shown that male rhesus can adapt to chronic 2G exposure. Body temperature showed a decrease in circadian amplitude during the first week at 2G, while the timing of the body temperature rhythm was delayed by over 1.5 hours. There were individual differences in performance at 2G; these ranged from no effect to a decrease of 50%. By collecting and analyzing daily urine samples, we were able to track the menstrual cycles of eight female rhesus. We then used these 8 animals to repeat our 1G and 2G studies. The female rhesus were able to complete the performance tasks with a high degree of accuracy and speed, as did the males. In addition, the females responded to 2G exposure in a similar manner to the males. However, there were significant changes in the circadian rhythms at different phases of the menstrual cycle. Future Directions We will continue our examination of mean levels and rhythms in performance in order to determine if these responses parallel those of other body rhythms. We will continue to study the response of the circadian rhythms of male and female rhesus to changes in the lighting and gravitational environment. KEY WORDS circadian rhythms FUNDING NASA NAG5-4320 PUBLICATIONS Barger, L.K., Takeuchi, P.T., and Fuller, C.A. 1998. Effects of menstrual cyclicity on circadian rhythms in female rhesus monkeys. FASEB Journal. 12(5):A749. Barger, L.K. and Fuller, C.A. 1998. Gender differences in the responses of rhesus monkeys to 2G. Gravitational & Space Biology Bulletin. 12(1):17.